A transfer case is often used in motor vehicles having all-wheel drive in order to distribute a torque, which is generated by the engine and is made available at a drive shaft of the transfer case, to two output shafts of the transfer case and, therefore, to more than one driven axle. Each of the output shafts, in this case, drives one axle of the motor vehicle or—if the motor vehicle has more than two driven axles—one axle drive with drive-through.
Along the power flow from the engine to the driven axle, the transfer case is usually installed downstream of the vehicle transmission transmitting the torque generated by the engine.
For example, for use in off-road vehicles and/or in heavy tractors, it is possible to provide, in addition to a multi-wheel drive and/or an all-wheel drive implemented by means of a transfer case, a two-stage transfer case, which is also referred to as an off-road reduction, having a first stage for road use and a second stage for off-road and/or heavy tractive use.
Such a two-stage intermediate gearing can be implemented, e.g., in the form of a planetary gear set, in which one or multiple stages can be locked relative to one another and/or relative to a stationary housing and/or relative to a shaft, for example by means of at least one claw clutch, whereby the transmission ratio changes.
Transfer cases with and without an intermediate gearing are classified as so-called differential-controlled transfer cases and clutch-controlled transfer cases, depending on their design as a permanent all-wheel drive or as an all-wheel drive which can be engaged and disengaged, e.g., automatically or manually by the driver of a motor vehicle.
In differential-controlled transfer cases, a differential gear, which can be locked manually, as necessary, or automatically, and/or a planetary gear distributes the power flow to two output shafts which are permanently coupled to the drive shaft.
In clutch-controlled transfer cases, an automatically and/or manually actuated clutch, in particular a friction clutch, such as, e.g., a lamellar clutch, or a viscous clutch with rotational-speed equalization or a rigid clutch without rotational-speed equalization, such as, e.g., a claw clutch, distributes the power flow to two driven shafts, one of which, i.e., the so-called primary shaft, is permanently coupled to the drive shaft, and the other of which, i.e., the so-called secondary shaft, is coupled to the drive shaft only as necessary and, otherwise, is decoupled from the drive shaft.
Transfer cases for an engageable and disengageable all-wheel drive are also known; although these transfer cases are not permanent, they comprise a differential and/or a planetary gear, which take or takes over the task of distributing the drive power when the all-wheel drive is engaged. In this hybrid form, the all-wheel drive is differential-controlled, although it can be disengaged and engaged by using one or multiple clutches. Since these transfer cases also comprise a clutch which can be engaged and disengaged via a secondary shaft, they are also considered to be clutch-controlled transfer cases.
For the sake of completeness, it should be noted that transfer cases are also known which operate without a clutch at all and without a differential or planetary gear, for example when the wheels have free-wheeling hubs on the driven axles.
Due to their simple and robust design, most clutch-controlled transfer cases have a friction clutch.
A friction clutch is a power-shiftable machine element for transferring torque between a driven shaft and a shaft to be driven.
A first typical embodiment of a friction clutch is a single-disk friction clutch, or a single-disk clutch. This comprises a first friction disk, which is also referred to as a clutch disk and which is non-rotatably connected to one of the two shafts, e.g., to the driven shaft, and a second friction disk, which is non-rotatably connected to the remaining shaft, e.g., to the shaft to be driven. The first friction disk, which forms the clutch disk, is disposed in such a way that the friction disk can be displaced with respect to the second friction disk along the common longitudinal axis of the two shafts.
If the friction disks of the driven shaft and the shaft to be driven are pressed together axially, a force-locked connection occurs and, therefore, torque is transferred and, when the driven shaft rotates, mechanical power is transferred.
A second typical embodiment of a friction clutch is a friction-lamellar clutch, or lamellar clutch.
Its characteristic feature, as compared to other friction clutches, such as, e.g., the previously described single-disk clutch, is the arrangement of multiple friction disks in a row, which friction disks are also referred to in that case as friction linings. In this case, a first friction disk, as viewed along the row, is non-rotatably connected to the driven shaft, a second friction disk is connected to the shaft to be driven, a third friction disk is also connected to the driven shaft, and so forth, in alternation. A friction disk which is non-rotatably connected to the driven shaft, and a friction disk which is non-rotatably connected to the shaft to be driven, form a friction pair. If the friction disks of the driven shaft and of the shaft to be driven are pressed together axially with a specified pressure force, the same pressure force acts on all friction pairs. Due to the larger contact area between the friction disks which are non-rotatably connected to the driven shaft and the friction disks which are non-rotatably connected to the shaft to be driven, higher torques can be transferred with the lamellar clutch as compared to, e.g., a single-disk clutch, given an identical pressure force.
Friction clutches in the form of lamellar clutches are used for torque transmission in conventional all-wheel drive trains. These clutches must be pressed together with a force of multiple kN, in order to transfer the required torques.
Friction clutches, such as, e.g., lamellar clutches, are mostly actuated electro-hydraulically, e.g., by means of an electric motor-driven hydraulic pump and a hydraulic piston, or electro-mechanically, e.g., by means of an electric motor via a spur-gear transmission and/or a worm gear and/or a ball ramp mechanism and/or a toggle lever mechanism and/or a combination thereof.
On the basis of the aforementioned, non-conclusive list, it is apparent that there is a multiplicity of different design possibilities for actuating a friction clutch.
Due to the usually highly limited installation space in clutch-controlled transfer cases having a two-stage intermediate gearing, some of the aforementioned actuating mechanisms are dispensed with at the start. In addition, due to the usually highly limited installation space and the high cost pressure, an installation of two actuating mechanisms, one of which actuates the clutch and the other of which shifts the intermediate gearing, is dispensed with, in principle.
In order to actuate a clutch-controlled transfer case having a two-stage intermediate gearing, it is therefore known to provide a device which both actuates the clutch and shifts the intermediate gearing.
A common solution of a device for actuating the clutch and for shifting the intermediate gearing comprises a relatively small 12 V electric motor in a power class of less than 100 W and having a torque of less than 1 Nm.
In light of these basic conditions, a correspondingly high mechanical transmission ratio between the electric motor and the clutch is required.
This transmission ratio must be reasonably priced, must have good efficiency, and must fit within a usually highly limited installation space.
A conventional embodiment of transfer cases having a two-stage intermediate gearing must both actuate the clutch and shift the two-stage intermediate gearing by means of a device having an electric motor-driven selector shaft having two cam disks.
In this case, in a first angle-of-rotation range of the selector shaft, the clutch is actuated in the first shifting stage of the intermediate gearing; in a second angle-of-rotation range of the selector shaft, shifting back and forth between the first and the second shifting stage occurs; and, in a third angle-of-rotation range of the selector shaft, the clutch is actuated in the second shifting stage of the intermediate gearing.
The entire angle range available therefor, which is 360°, is limited to one full revolution of the selector shaft. Therefore, the transmission ratio for actuating the clutch is also limited.
A clutch-controlled transfer case having a two-stage intermediate gearing and a clutch designed as a lamellar clutch is known from DE 11 2006 002 138 T5. For the purpose of actuating the clutch and shifting the intermediate gearing, the transfer case comprises a device having a selector shaft and an electric motor, which sets the selector shaft into rotation, and two cam disks, one clutch cam disk, and one selector cam disk. The selector cam disk is disposed on the selector shaft. A rotation of the selector cam disk starting from a neutral position by, at most, one half of a revolution in both directions of rotation results in an axial motion which, depending on the direction of rotation of the selector shaft, effectuates a shifting of the intermediate gearing from the first stage into the second stage and vice versa. The clutch cam disk is connected to a ball ramp mechanism, which acts parallel to the longitudinal axis of the selector shaft and acts axially on the lamellar clutch. The ball ramp mechanism presses the lamellar clutch together when the clutch cam disk is rotated, starting from a neutral position, both in a first direction of rotation and in an opposite, second direction of rotation. The clutch cam disk is rotatably driven by means of the selector shaft. For this purpose, a disk cam having two entraining elements, which are spaced apart from one another by a neutral angle of rotation, is disposed on the selector shaft and engages with the clutch cam disk when the selector shaft is rotated both in a first direction of rotation and in an opposite, second angle of rotation, starting from a neutral position. The neutral angle of rotation in this case is dimensioned in such a way that, starting from a neutral position of the selector shaft, one of the two entraining elements, depending on the direction of rotation, finally entrains the clutch cam disk when half the neutral angle of rotation has been exceeded and, therefore, swivels the clutch cam disk in the direction opposite to the selector shaft. As a result, shifting back and forth between the shifting stages of the intermediate gearing can occur, by means of the selector cam disk, within the angle-of-rotation range given by the neutral angle of rotation, whereupon the lamellar clutch is pressed together only when the clutch cam disk is entrained by one of the two entraining elements. The clutch cam disk experiences a rotation by only approximately 45° in each direction, in this case, given an angle-of-rotation range of the selector shaft, starting from its neutral position, which is limited, overall, to one full revolution, as described above in the description of the selector cam disk. As a result, said clutch cam disk experiences a maximum angle-of-rotation range of one-fourth of a revolution as compared to one full revolution of the selector shaft. As a result, the entire transmission ratio of the ball ramp mechanism disadvantageously tends toward one-fourth of one revolution, both in one direction of rotation and in the other direction of rotation. Alternatively, the ball ramp mechanism can be directly coupled to the selector shaft via a gear set. The clutch cam disk and the disk cam are dispensed with in this case. For the same mode of operation, the ball ramp mechanism requires a neutral range, however, within which the switching process can take place.
It is known from EP 1 977 128 B1, with respect to a clutch-controlled transfer case having a two-stage intermediate gearing and a clutch designed as a lamellar clutch, to design the clutch cam disk of a device for actuating the clutch and for shifting the intermediate gearing so as to have a selector shaft and an electric motor, which sets the selector shaft into rotation, and two cam disks, one selector cam disk, and one clutch cam disk, having two helical channels, each of which spirals once around the clutch cam disk axis, in and along each of which one of two rollers roll, which rollers are disposed diametrically opposite one another with respect to the clutch cam disk axis. Each helical channel has a central depression in its center, starting from which a ramp, which rises in the axial direction, extends toward each of the opposite ends of each helical channel, in both directions of rotation. As viewed from a center point of the clutch cam disk, which is situated on the clutch cam disk axis, the two depressions of the two helical channels are disposed diametrically opposite one another. The rollers are rotatable about radially extending axes and are disposed so as to be radially displaceably supported in a gear, which meshes with a spur gear and is rotatable via the spur gear by means of the selector shaft. The clutch disk itself is disposed between two stops, which define a neutral range, so as to be rotatable by approximately 180°. In the neutral position of the selector shaft, the clutch disk is situated in an angle-of-rotation position between the two stops. The selector cam disk is non-rotatably connected to the selector shaft. The selector cam disk has an axial gate guide. The gate guide has a ramp around the neutral position, which ramp rises in the axial direction along the selector shaft and, at each of its two ends, transitions into a slot, which encircles the selector shaft, in a plane which is normal to the axis of the selector shaft. A selector pin, which is guided by means of the gate guide in both axial directions along the selector shaft, engages into the gate guide. A movement of the selector pin parallel to the axis of the selector shaft effectuates a change in the shifting stages of the intermediate gearing. If the selector shaft rotates, starting from the neutral position, in one direction of rotation, the selector shaft drives the gear, via the spur gear, using both rollers. When the selector shaft is rotated, starting from the neutral position, in one direction of rotation, the two rollers of the gear coupled to the selector shaft—which rollers are situated, in the neutral position, between the oppositely rising ramps of the helical channel, in the depressions assigned to the rollers—rotate the clutch cam disk out of the neutral position and against one of the two stops, depending on the direction of rotation. In order to rotate the clutch cam disk from stop to stop, the selector shaft rotates through one angle-of-rotation range referred to as a neutral range, within which shifting back and forth between the first and the second shifting stage of the intermediate gearing can occur without actuating the clutch. The shifting back and forth between the first and the second shifting stage of the intermediate gearing takes place by way of the selector shaft simultaneously rotating the selector cam disk during the rotation of the clutch cam disk out of the neutral position and against one of the stops, in the gate guide of which selector cam disk the selector pin initially follows the ramp and thereby experiences an axial displacement. Rotating the selector shaft in the opposite direction effectuates a switch between the shifting stages of the intermediate gearing. If the selector shaft continues to rotate in a direction of rotation that has been selected, the ramp of the gate guide of the selector cam disk transitions at the end of the neutral range into the slot, and therefore the selector pin does not experience any further axial displacement and the selected shifting stage of the intermediate gearing is retained. The two rollers of the gear, which also continues to rotate as the selector shaft continues to rotate, however, follow the ramps, which rise in the axial direction starting from the two central depressions, in the two helical channels, each of which is associated with one of the two rollers, of the clutch cam disk, which is now resting against a stop. The rollers are part of a ball ramp mechanism, which presses the lamellar clutch together with force which increases, the further the rollers follow the ramps upward in the helical channels associated with the rollers. Due to the radial displaceability of the rollers, a rotation of the gear with respect to the clutch cam disk, which is resting against a stop, by one full revolution is made possible. As a result, the transmission ratio necessary for pressing the lamellar clutch together is advantageously distributed across one full revolution in each direction. In addition, the neutral range, within which a switch between the shifting stages of the intermediate gearing takes place, can be defined by a suitable position of the stops. As a result, the transmission for switching the shifting stages can also be distributed across the neutral range defined by the stops.
It is known from EP 1 875 109 B1, with respect to a clutch-controlled transfer case having a friction clutch, which is designed as a lamellar clutch, e.g., to obtain a high transmission ratio for the actuation of the friction clutch by pressing the friction clutch together by means of two ramp rings. It is essential that a rotation of the two ramp rings in the opposite direction pushes the ramp rings apart in the axial direction. One scissor lever is mounted on each of the ramp rings or is integral therewith. The ends of the scissor lever provided with rollers ride on the circumference of a clutch cam disk. The circumference of the clutch cam disk is bisected into two curved paths, which are point-symmetrical with respect to the center point of the clutch cam disk, with one curved path for each scissor lever. The curved paths are designed in such a way that the scissor levers swivel in opposite directions with increasing rotation of the clutch cam disk starting from a starting position. As a result, the transmission ratio necessary for pressing the lamellar clutch together can be distributed across one half of one revolution of the clutch cam disk in one direction of rotation.
A feature shared by the clutch-controlled transfer cases having a two-stage intermediate gearing known from the prior art is a limited transmission ratio for pressing their friction clutches together.